This invention relates to a continuously hot dipped metallic coated ferritic chromium alloy ferrous base strip and a process to enhance the wetting of the strip surface with molten aluminum.
Hot dip aluminum coated steel exhibits a high corrosion resistance to salt and finds various applications in automotive exhaust systems and combustion equipment. In recent years, exhaust system requirements have increased with respect to durability and aesthetics. For this reason, there has become a need to increase high temperature oxidation resistance and salt corrosion resistance by replacing aluminum coated low carbon or low alloy steels with aluminum coated chromium alloy steels. For high temperature oxidation, at least part of the aluminum coating layer can be diffused into the iron base by the heat during use to form an Fe-Al alloy layer. If uncoated areas are present in the aluminum coating layer, accelerated oxidation leading to a perforation of the base metal may result if the Fe-Al alloy is not continuously formed on the base metal. For lower temperatures, the aluminum coating layer acts as a barrier protection for atmospheric conditions and as a cathodic coating in high salt environments. Again, if uncoated areas are present, accelerated corrosion may occur leading to failure of the coated structure.
It is well known to hot dip metallic coat low carbon steel strip without a flux by subjecting the strip to a preliminary treatment which provides a clean surface free of oil, dirt and iron oxide which is readily wettable by the coating metal. One type of preliminary in-line anneal treatment for low carbon steel is described in U.S. Pat. No. 3,320,085 issued to C. A. Turner, Jr. The Turner process, also known as the Selas process, for preparation of low carbon steel strip for hot dip metallic coating includes passing the strip through a direct fired furnace having an atmosphere heated to a temperature of at least 2400.degree. F. (1316.degree. C.). The atmosphere is formed from the gaseous products of combustion of fuel and air and has no free oxygen. The fuel-air ratio is controlled to provide the necessary reducing characteristics for effecting cleaning of the steel strip. The fuel-air ratio is regulated to provide a slight excess of fuel so that there is no free oxygen but excess combustibles in the form of carbon monoxide and hydrogen. Maintaining a furnace atmosphere of at least 1316.degree. C. having at least 3% excess combustibles is reducing to steel up to 1700.degree. F. (927.degree. C.). Turner teaches his cleaned strip is then passed through a sealed delivery duct having a neutral or protective atmosphere prior to passing the cleaned strip into a coating pot. For coating with molten zinc, Turner teaches heating the strip up to 1000.degree. F. (538.degree. C.). For coating with molten aluminum, Turner teaches heating the strip within the temperature range of 1250.degree.-1300.degree. F. (677.degree.-704.degree. C.) in the direct fired furnace since the atmosphere is still reducing to the steel at these temperatures.
Modern direct fired furnaces include an additional furnace section normally heated with radiant tubes. This furnace section contains the same neutral or reducing protective atmosphere, e.g. 75% nitrogen-25% hydrogen, as the delivery duct described above.
U.S. Pat. No. 3,925,579 issued to C. Flinchum et al describes an in-line pretreatment for hot dip aluminum coating low alloy steel strip to enhance wettability by the coating metal. The steel contains one or more of up to 5% chromium, up to 3% aluminum, up to 2% silicon and up to 1% titanium, all percentages by weight. The strip is heated to a temperature above 1100.degree. F. (593.degree. C.) in an atmosphere oxidizing to iron to form a surface oxide layer, further treated under conditions which reduce the iron oxide whereby the surface layer is reduced to a pure iron matrix containing a uniform dispersion of oxides of the alloying elements.
The problems associated with nonwetting of aluminum coatings onto ferritic chromium alloy steel are also well known. Hot dip aluminum coatings have poor wettability to ferritic chromium alloy steel base metals and normally have uncoated or bare spots in the aluminum coating layer. By poor adherence is meant flaking or crazing of the coating during bending the strip. To overcome the adherence problem, some have proposed heat treating the aluminum coated steel to anchor the coating layer to the base metal. Others lightly reroll the coated chromium alloy steel to bond the aluminum coating. Finally, those concerned about uncoated spots have generally avoided continuous hot dip coating. Rather, batch type hot dip coating or spray coating processes have been used. For example, after a chromium alloy steel article has been fabricated, it is dipped for an extended period of time within an aluminum coating bath to form a very thick coating layer.
U.S. Pat. No. 4,675,214 issued to F. M. Kilbane et al, incorporated herein by reference, proposes a solution for enhancing the wetting of ferritic chromium alloy steel strip continuously coated with hot dip aluminum coatings. The Kilbane process includes cleaning a ferritic chromium alloy steel and passing the cleaned steel through a protective hydrogen atmosphere substantially void of nitrogen prior to entry of the steel into an aluminum coating bath. This process resulted in improved wetting of ferritic chromium alloy steel so long as the steel was not cleaned by heating to an elevated temperature in a direct fired furnace. According to Turner, a direct fired furnace having an atmosphere with at least 3% combustibles heated to 2400.degree. F. (1316.degree. C.) is reducing to steel up to 1700.degree. F. (927.degree. C.). Nevertheless, heating ferritic chromium alloy steel at temperatures about 1250.degree. F. (677.degree. C.) and above in a direct fired furnace whose atmosphere has no free oxygen and subsequently passing the steel through a protective atmosphere of substantially pure hydrogen immediately prior to hot dip coating with aluminum still had large uncoated areas. Not being bound by theory, it is believed a direct fired furnace atmosphere having no free oxygen does have significant oxidizing potential due to the presence of water and apparently is oxidizing to the chromium contained in a chromium alloy ferrous strip. The chromium oxide formed on the surface of the strip apparently is not removed sufficiently by the protective hydrogen atmosphere prior to entry into the coating bath thereby preventing complete wetting of the strip surface.